System and method for improving CMOS compatible non volatile memory retention reliability

ABSTRACT

A system and method is disclosed for improving complementary metal oxide semiconductor (CMOS) compatible non volatile memory (NVM) retention reliability in memory cells. A memory cell of the invention comprises a backend layer that reduces charge leakage from a floating gate of the memory cell. A first bottom portion of the backend layer is formed from a first layer of silicon oxynitride having a low value of defect/trap density. A second top portion of the backend layer is formed from a second layer of silicon oxynitride having a high value of defect/trap density. The first layer of silicon oxynitride inhibits electron transport and the second layer of silicon oxynitride protects CMOS devices from plasma induced damage.

TECHNICAL FIELD OF THE INVENTION

The present invention is generally directed to the manufacture ofintegrated circuits and, in particular, to a system and method forproviding an improvement in CMOS compatible non volatile memoryretention reliability.

BACKGROUND OF THE INVENTION

A fundamental design challenge in creating a memory cell of anelectrically erasable programmable read only memory (EEPROM) device isto use a controllable and reproducible electrical effect that hassufficient non-linearity so that the memory cell (1) can be written to(or erased) at one voltage in less than one millisecond (1 ms) and canbe read at another voltage, and (2) the data within the memory cell mustremain unchanged for more than ten (10) years.

Prior art stacked/split gate EEPROM technology requires (1) specialmulti-polysilicon materials, (2) different gate oxide thicknesses, and(3) modified doping profiles. These prior art requirements createprocess complexity and high cost when embedded into a complementarymetal oxide semiconductor (CMOS) process.

It is well known that CMOS compatible non volatile memory (NVM) devicesoften experience additional charge leakage through backend dielectriclayers. The charge leakage through backend dielectric layers is inaddition to the charge leakage that NVM devices experience through thefront end gate oxide layer in stacked gate NVM devices.

FIG. 1 illustrates a schematic cross sectional diagram of an exemplaryprior art non volatile memory (NVM) cell 100. Memory cell 100 comprisesa polysilicon gate 110 (designated “POLY GATE 110” in FIG. 1).Polysilicon gate 110 and other elements of the memory cell 100 arecovered with backend dielectric 120. In this example backend dielectric120 is made of tetraethyloxysilane (TEOS). Therefore, backend dielectric120 is designated as “TEOS 120” in FIG. 1.

Backend dielectric TEOS 120 is covered with backend dielectric 130. Inthis example backend dielectric 130 is made of silicon oxynitride(SiON). Therefore, backend dielectric 130 is designated “SiON 130” inFIG. 1. Lastly, backend dielectric 130 is covered with backenddielectric 140. In this example backend dielectric 140 is made of plasmaenhanced chemical vapor deposition (PECVD) oxide. Therefore, backenddielectric 140 is designated “PECVC oxide 140” in FIG. 1.

The fabrication process for a single poly NVM device is compatible withCMOS devices. The polysilicon gate in an NVM device is a floating gate(FG) that store electrons. The backend dielectric layers (TEOS 120, SiON130, PECVD oxide 140) have a higher density of defects/traps than afront end gate oxide layer. The higher density of defects/traps enhancesthe Trap-Assisted-Tunneling (TAT) of electrons in the oxide layers. Thehigher density of defects/traps enhances thermal excitation in thesilicon oxynitride (SiON) layer.

In a CMOS process the silicon oxynitride (SiON) layer is designed to beleaky (i.e., to have more defects/traps) in order to prevent plasmainduced damage. Electrons stored on the polysilicon floating gate 110may tunnel through the TEOS layer 120 to arrive at the leaky siliconoxynitride layer 130 (SiON 130). From the SiON layer 130 the electronscan easily move to the substrate layer (not shown in FIG. 1). Thisresults in a higher charge decay rate for electrons that are stored onthe floating gate. This also results in retention degradation especiallyat higher temperatures.

FIG. 2 illustrates an energy band diagram 200 for the electron chargedecay mechanism (i.e., backend charge leakage). The electrons(designated e− in FIG. 2) are initially located on the polysiliconfloating gate (FG) 110. The electrons can leak out of the floating gate(FG) 110 through the TEOS dielectric layer 120 throughTrap-Assisted-Tunneling (TAT). This is shown in FIG. 2 by an electronentering one of the traps in the TEOS layer 120. The electrons then passinto the silicon oxynitride (SiON) layer 130. As shown in FIG. 1 and inFIG. 2 the electrons move through the silicon oxynitride (SiON) layer130 to the substrate (not shown in FIG. 1).

This backend charge leakage is a serious problem for CMOS compatible NVMdevices. Several different methods have been explored to reduce theleakage of charge from the backend dielectric layers in order to improveCMOS compatible retention performance.

A first approach has been to add a mask to the silicon oxynitride (SiON)layer 130 to etch the silicon oxynitride (SiON) layer 130 away from thetop of the floating gate (FG) 110. The mask allows the SiON layer 130 tobe selectively etched from the top of the NVM devices. The SiON layer130 is left on top of the CMOS devices. The absence of SiON layer 130over the top of the floating gate (FG) 110 breaks the charge leakagepath. Electrons on the floating gate (FG) 110 therefore have a muchslower decay rate by tunneling in oxide instead of going through arelatively leaky SiON layer 130 that is connected to the substrate.

A second approach has been to increase the thickness of the TEOS layer120. This increases the tunneling distance from the floating gate (FG)110 to the SiON layer 130. This causes the electrons to take a longertime to arrive at the relatively leaky SiON layer 130.

A third approach has been to modulate the density of the defects/trapsin the SiON layer 120 in order to make the SiON layer 120 highlynon-conductive. The fewer defects/traps there are in the SiON layer 120,the less conductivity there is in the SiON layer 120. Electrons thatarrive at the SiON layer 120 will have difficulty in traveling to otherlocations. The electrons that collect in the SiON layer 120 will deterfurther leakage of electrons from the floating gate (FG) 110.

CMOS compatible NVM performance is not as good as other mainstream NVMtechnology (e.g., stacked gate NVM, split gate NVM, SONOS(silicon-oxide-nitride-oxide-silicon) NVM). CMOS compatible NVMperformance has slow speed, low density and limited endurance. But ithas one major advantage. It has low cost because its fabrication processis compatible with CMOS processes. CMOS compatible NVM technology isadvantageous in some applications where limited speed, density andendurance NVM technology is needed but the cost would be too high ifmainstream NVM technology were employed. Therefore, the low cost featureis critical to CMOS compatible NVM technology.

The three approaches mentioned above for improving CMOS compatible NVMretention performance have significant drawbacks. The first approachrequires the addition of a mask and etch step to the standard CMOSprocess. This increases the cost. More importantly, when etching theSiON layer 130 from the top of the floating gate (FG) 110, limited etchselectivity between the SiON layer 130 and the TEOS layer 120 alwaysleads to some over-etch of the TEOS layer 120. A thinner TEOS layer 120will lead to an increased level of electron discharge from the floatinggate (FG) 110. In addition, etching away the SiON layer 130 will exposethe floating gate (FG) 110 and the TEOS layer 120 to backend plasmadamage. This will lead to a degradation of the NVM device retention.

The second approach requires an increase in the thickness of the TEOSlayer 120. In order to get a satisfactory retention performance the TEOSlayer 120 needs to have at least a double thickness. The creation of athicker TEOS layer 120 alters the CMOS process and significantly altersthe device parameters. After this step the NVM device is no longer CMOScompatible.

The third approach results in plasma induced damage. The CMOS processrequires that the SiON layer 130 be leaky so that charges from theplasma process have a leakage path.

Therefore, there is a need in the art for a system and a method that can(1) reduce charge leakage from the floating gate, and (2) not result inplasma induced damage, and (3) not significantly change CMOS deviceparameters. There is a need in the art for a system and a method thatcan balance the NVM retention requirements and the CMOS devicerequirements. There is a need in the art for a system and a method thatcan fabricate NVM devices and CMOS devices in the same manufacturingprocess.

Before undertaking the Detailed Description of the Invention below, itmay be advantageous to set forth definitions of certain words andphrases used throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like.

Definitions for certain words and phrases are provided throughout thispatent document, those of ordinary skill in the art should understandthat in many, if not most instances, such definitions apply to prioruses, as well as to future uses, of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 illustrates a schematic cross sectional diagram of an exemplaryprior art non volatile memory (NVM) cell;

FIG. 2 illustrates an energy band diagram for an electron charge decaymechanism from a floating gate of the prior art non volatile memory(NVM) cell shown in FIG. 1;

FIG. 3 illustrates an amphoteric trap model that shows the chemicalstructure of a dangling silicon (Si) bond in a nitride film;

FIG. 4A illustrates a first advantageous embodiment of the presentinvention in which a silicon oxynitride (SiON) layer comprises twoseparate layers each having a different value of a defect/trap density;

FIG. 4B illustrates a second advantageous embodiment of the presentinvention in which a silicon oxynitride (SiON) layer comprises one layerhaving a defect/trap density gradient;

FIG. 5 illustrates a schematic cross sectional diagram of anadvantageous embodiment of a non volatile memory (NVM) cell of thepresent invention;

FIG. 6 illustrates a graph that shows metal antenna gate oxide leakagefor non volatile memory cells with a standard silicon oxynitride (SiON)dielectric and for non volatile memory cells of the present inventionthat have a silicon rich silicon oxynitride (SiON) dielectric;

FIG. 7 illustrates a non volatile memory cell showing that silicon richsilicon oxynitride (SiON) can reduce plasma induced damage by proving analternate conductive path for plasma induced charge; and

FIG. 8 illustrates a graph that shows retention times for non volatilememory cells with and without a silicon oxynitride (SiON) dielectric.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 3 through 8, discussed below, and the various embodiments used todescribe the principles of the present invention in this patent documentare by way of illustration only and should not be construed in any wayto limit the scope of the invention. Those skilled in the art willunderstand that the principles of the present invention may beimplemented with any type of suitably arranged non volatile memory (NVM)technology.

The present invention comprises an improved non volatile memory (NVM)cell. The non volatile memory (NVM) cell of the present inventioncomprises a second silicon oxynitride (SiON) layer on top of a firstsilicon oxynitride (SiON) layer. The first and second SiON layers arelocated over a TEOS layer and the TEOS layer is located over thefloating gate (FG) of the memory cell.

The second SiON layer (i.e., the top SiON layer) has a higherdefect/trap density than does the first SiON layer (i.e., the bottomSiON layer). In an advantageous embodiment of the invention the combinedthickness of the first SiON layer and the second SiON layer equals thethickness of a prior art SiON layer (e.g., SiON layer 130 in memory cell100).

The first (bottom) SiON layer inhibits electron transport (with itsrelatively low defect/trap density) and improves NVM retention. Thesecond (top) SiON layer (with its relatively high defect/trap density)protects the CMOS devices from plasma induced damage. The non volatilememory (NVM) cell of the present invention improves CMOS compatible NVMretention with a minimum alteration of the CMOS device manufacturingprocess.

The method of the present invention creates a double SiON layer byforming the first (bottom) SiON layer over the TEOS layer and thenforming the second (top) SiON layer over the first (bottom) SiON layer.The first (bottom) SiON layer is fabricated using a low silane (SiH₄) toammonia (NH₃) gas flow ratio. Typically a one to sixteen ratio is used.That is, the gas flow comprises one part silane (SiH₄) to sixteen partsammonia (NH₃) to fabricate the first (bottom) SiON layer.

The second (top) SiON layer is fabricated using a high silane (SiH₄) toammonia (NH₃) gas flow ratio. Typically a one to three ratio is used.That is, the gas flow comprises one part silane (SiH₄) to three partsammonia (NH₃) to fabricate the second (top) SiON layer. In anadvantageous embodiment of the invention the double SiON layer isfabricated in one process step by controlling the gas flow ratio for thetwo different SiON layers at different times.

In an alternate embodiment of the invention, the SiON layer isfabricated by depositing a single SiON layer that has a defect/trapdensity gradient. The defect/trap density gradient extends from thebottom of the SiON layer (where there is a relatively low defect/trapdensity) to the top of the SiON layer (where there is a relatively highdefect/trap density). In this advantageous embodiment there is one SiONlayer with a defect/trap density gradient instead of two SiON layerseach with a different defect/trap density value.

In a standard CMOS process the silicon oxynitride (SiON) is fabricatedby a chemical vapor deposition (CVD) process. The basic chemicalreaction is as follows:SiH₄+NH₃+H₂O→SiON+4H₂+H⁺  (1)

Silicon atoms create dangling bonds in SiON film. Hydrogen atomsterminate the dangling bonds by forming silicon-hydrogen (Si—H) bonds.The ratio of the silane (SiH₄) to ammonia (NH₃) in the gas flowdetermines the defect/trap density in the silicon oxynitride (SiON)film.

FIG. 3 illustrates a physical model (referred to as an “amphoteric trapmodel”) that shows the chemical structure of a dangling silicon (Si)bond in a nitride film. As shown in FIG. 3, each silicon (Si) atom has avalence of four. Each of three of the four valence bonds is connected toa nitrogen (N) atom. If the fourth valence bond is unoccupied it isreferred to as a “dangling” silicon (Si) bond. This state is shown inFIG. 3 with the designation D.

In a standard CMOS process a one to six (1:6) gas flow ratio istypically used for the silane (SiH₄) to ammonia (NH₃) to create asilicon oxynitride (SiON) film above the polysilicon gate and TEOSdielectric layer. For flash memories built on CMOS processes a siliconoxynitride (SiON) film with a one to sixteen (1:16) gas flow ratio forthe silane (SiH₄) to ammonia (NH₃) has been developed for interpolyisolation because of its relatively low defect/trap density. Inaddition, a silicon oxynitride (SiON) film with a one to three (1:3) gasflow ratio for the silane (SiH₄) to ammonia (NH₃) has been developed forcharge storage purposes because of its relatively high defect/trapdensity.

FIG. 4A illustrates an advantageous embodiment of the present inventionin which a silicon oxynitride (SiON) layer is deposited in two steps. Inthe first step of forming the SiON layer, a low defect/trap density SiONlayer 410 is deposited. This is accomplished by setting the silane(SiH₄) to ammonia (NH₃) gas flow ratio to one to sixteen (1:16) duringthe first half of the SiON fabrication process. Low defect/trap densitySiON layer 410 has a correspondingly low conductivity. The lowdefect/trap density inhibits electron transport in the SiON layer 410and inhibits the ultimate leakage of the electrons to the substrate (notshown). If electrons from the floating gage (FG) tunnel through the TEOSlayer and enter the SiON layer the electrons will first encounter thelow conductivity SiON layer 410.

In the second step of forming the SiON layer, a high defect/trap densitySiON layer 420 is deposited over the low defect/trap density SiON layer410. This is accomplished by setting the silane (SiH₄) to ammonia (NH₃)gas flow ratio to one to three (1:3) during the second half of the SiONfabrication process. The high defect/trap density SiON layer 420 has acorrespondingly high conductivity. The high defect/trap density of SiONlayer 430 effectively protects the CMOS devices from plasma induceddamage.

In one advantageous embodiment the thickness of the low density/trapSiON layer 410 and the thickness of the high density/trap SiON layer 420are each equal to one half of the thickness of a prior art SiON layer(e.g., SiON layer 130 of prior art memory cell 100). In this embodimentthe combined thickness of the SiON layer 410 and the SiON layer 420equals the thickness of a prior art SiON layer.

The second SiON layer 420 is intentionally deposited with a higher gasflow ratio (e.g., one to three (1:3)) than the gas flow ratio normallyused in CMOS processes (e.g., one to six (1:6). This is for theconsideration of other roles that the silicon oxynitride (SiON) filmsplay in CMOS processes that are related to SiON film properties (e.g.,refractive index, ultraviolet (UV) absorption coefficient,permittivity). The combined effect of the bottom SiON layer 410 and thetop SiON layer 420 is similar to an original single layer prior art SiONlayer in terms of such film properties.

FIG. 4B illustrates another advantageous embodiment of the presentinvention in which a single SiON layer 430 is deposited. SiON layer 430is deposited so that the value of the defect/trap density continuouslyincreases from a relatively low value at the bottom of the SiON layer430 to a relatively high value at the top of the SiON layer 430. Thatis, the defect/trap density has a gradient that extends from the bottomto the top of the SiON layer 430. In this advantageous embodiment thereis one SiON layer 430 with a defect/trap density gradient instead of twoSiON layers (410, 420) each with a different defect/trap density value.To create the SiON layer 430 the ratio of the silane (SiH₄) to ammonia(NH₃) gas flow is continuously increased from one to sixteen (1:16) atthe bottom of SiON layer 430 to one to three (1:3) at the top of SiONlayer 430 during the deposition process.

The fabrication of the SiON layer 410 and the SiON layer 420 can beaccomplished in one fabrication step by controlling the gas flow ratioat different times. The fabrication of the SiON layer 430 can beaccomplished in one fabrication step by continuously changing the gasflow ratio during the deposition time interval.

FIG. 5 illustrates a schematic cross sectional diagram of anadvantageous embodiment of a non volatile memory (NVM) cell 500 of thepresent invention. As shown in FIG. 5, memory cell 500 comprises apolysilicon gate 510 (designated “POLY GATE 510” in FIG. 5). Polysilicongate 510 and other elements of the memory cell 500 are covered withbackend dielectric 520. In this example backend dielectric 520 is madeof tetraethyloxysilane (TEOS). Therefore, backend dielectric 520 isdesignated as “TEOS 520” in FIG. 5.

Backend dielectric TEOS 520 is covered with a first silicon oxynitride(SiON) layer 410. First silicon oxynitride (SiON) layer 410 isdesignated “SiON 410” in FIG. 5. As previously described, SiON layer 410has a low defect/trap density. First SiON layer 410 is covered with asecond silicon oxynitride (SiON) layer 420. Second silicon oxynitride(SiON) layer 420 is designated “SiON 420” in FIG. 5. As previouslydescribed, SiON layer 420 has a high defect/trap density.

Lastly, SiON layer 420 is covered with backend dielectric 540. In thisexample backend dielectric 540 is made of plasma enhanced chemical vapordeposition (PECVD) oxide. Therefore, backend dielectric 540 isdesignated “PECVC oxide 540” in FIG. 5.

The non volatile memory (NVM) cell 500 of the present invention has goodNVM retention. In addition NVM cell 500 can be fabricated together withstandard CMOS devices without additional cost.

One of the design goals in CMOS compatible NVM technology is to improvethe NVM retention reliability without negatively impacting CMOS deviceperformance and without increasing the costs. The approach of thepresent invention circumvents this problem by modulating the SiONdefect/trap density at different physical locations. Because the presentinvention does not need to etch the SiON layer the present inventioncompletely avoids the problem of over-etching the underlying TEOS layer.

Experimental tests of the non volatile memory (NVM) cell 500 of thepresent invention have been performed. The performance of the NVM cell500 was compared with the performance of a prior art memory cell 100. Itwas found that the silicon rich silicon oxynitride (SiON) layer 420helps protect the CMOS devices from plasma induced damage.

FIG. 6 illustrates a graph 600 that shows metal antenna gate oxideleakage for prior art non volatile memory cells with a standard siliconoxynitride (SiON) layer. Graph 600 also shows metal antenna gate oxideleakage for non volatile memory cells of the present invention that havea silicon rich silicon oxynitride (SiON) layer 420. The abscissa ofgraph 600 illustrates the leakage current in amperes. The ordinate ofgraph 600 illustrates the cumulative probability.

As shown in graph 600, the leakage current for the non volatile memorycells of the present invention in on the order of 10⁻¹² amperes. Theleakage current for the prior art non volatile memory cells with astandard silicon oxynitride (SiON) layer is significantly larger thanthe leakage current for the non volatile memory cells of the presentinvention.

The silicon rich silicon oxynitride (SiON) layer 420 of the presentinvention has a higher defect/trap density and higher conductivity. Thesilicon rich silicon oxynitride (SiON) layer 420 provides an alternateconductive path for plasma induced charge. The alternate conductive pathis illustrated in FIG. 7.

FIG. 7 illustrates a non volatile memory cell 700 showing that siliconrich silicon oxynitride (SiON) layer 420 can reduce plasma induceddamage by proving an alternate conductive path for plasma inducedcharge. Charge flows from metal antenna 710 through conductor 720.Conductor 720 is connected to the SiON layer 420. The charges inconductor 720 enter the SiON layer 420 and flow laterally through theSiON layer 420.

An experiment was also conducted to determine the effect of siliconoxynitride (SiON) on non volatile memory retention. The results of theexperiment are shown in graph 800 in FIG. 8. Graph 800 shows retentiontimes for non volatile memory cells with and without a siliconoxynitride (SiON) layer.

If the TEOS layer is not deposited, then the SiON layer will be indirect contact with the floating gate of the NVM device. The experimentshowed that in this case the NVM device will lose all the charge storedon the floating gate in two hundred sixty (260) hours at a temperatureof two hundred twenty five degrees Centigrade (225° C.) (when thethreshold voltage window (Vth) drops to zero).

But if the TEOS layer is deposited and the SiON layer is not depositedduring the fabrication of the NVM device, then the retention at thetemperature of two hundred twenty five degrees Centigrade (225° C.) ismuch better. Although leaving out the SiON layer improves NVM retentionit is unacceptable to leave out the SiON layer because the absence ofthe SiON layer exposes the CMOS devices to plasma damage.

These experimental results indicate that the system and method of thepresent invention is capable of improving non volatile memory (NVM)retention reliability without having an adverse effect on CMOS devices.The system and method of the present invention allows NVM devices andCMOS devices to be fabricated in the same process. With the presentinvention there is no additional cost when NVM devices are embedded in aCMOS process.

The foregoing description has outlined in detail the features andtechnical advantages of the present invention so that persons who areskilled in the art may understand the advantages of the invention.Persons who are skilled in the art should appreciate that they mayreadily use the conception and the specific embodiment of the inventionthat is disclosed as a basis for modifying or designing other structuresfor carrying out the same purposes of the present invention. Persons whoare skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the inventionin its broadest form.

Although the present invention has been described with an exemplaryembodiment, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present invention encompasssuch changes and modifications as fall within the scope of the appendedclaims.

1. A method for reducing charge leakage through a layer formed on top ofa polysilicon gate of a memory cell, said method comprising the stepsof: forming a bottom portion of said layer of said memory cell from afirst layer of silicon oxynitride; and forming a top portion of saidlayer of said memory cell from a second layer of silicon oxynitride;wherein the second layer of silicon oxynitride has a higher defect/trapdensity than does the first layer of silicon oxynitride.
 2. The methodas set forth in claim 1 wherein said first layer of silicon oxynitrideinhibits electron transport through the first layer of siliconoxynitride.
 3. The method as set forth in claim 1 wherein said secondlayer of silicon oxynitride provides protection from plasma induceddamage.
 4. The method as set forth in claim 1 wherein said step offorming said bottom portion of said layer comprises: forming adefect/trap density in said first layer of silicon oxynitride using asilane to ammonia gas flow comprising approximately one part silane toapproximately sixteen parts ammonia.
 5. The method as set forth in claim1 wherein said step of forming said top portion of said layer comprises:forming a defect/trap density in said second layer of silicon oxynitrideusing a silane to ammonia gas flow comprising approximately one partsilane to approximately three parts ammonia.
 6. A method for improvingcomplementary metal oxide semiconductor (CMOS) compatible non-volatilememory retention reliability in a memory cell, said method comprisingthe step of: forming a layer on top of a polysilicon gate of said memorycell, wherein the layer has a bottom portion comprising a first layer ofsilicon oxynitride and a top portion comprising a second layer ofsilicon oxynitride; wherein the second layer of silicon oxynitride has ahigher defect/trap density than does the first layer of siliconoxynitride.
 7. The method as set forth in claim 6 wherein said firstlayer of silicon oxynitride inhibits electron transport through thefirst layer of silicon oxynitride and wherein said second layer ofsilicon oxynitride protects CMOS devices from plasma induced damage. 8.The method as set forth in claim 6 wherein said formation of saidbackend layer for said memory cell does not change CMOS deviceparameters.
 9. The method as set forth in claim 6 wherein: said firstlayer of silicon oxynitride has a defect/trap density formed by using alow silane to ammonia gas flow of approximately one part silane toapproximately sixteen parts ammonia; and said second layer of siliconoxynitride has a defect/trap density formed by using a high silane toammonia gas flow of approximately one part silane to approximately threeparts ammonia.
 10. The method as set forth in claim 6 wherein the firstlayer of silicon oxynitride has a defect/trap density gradient thatextends from a first defect/trap density at a bottom of the first layerof silicon oxynitride to a second defect/trap density at a top of thefirst layer of silicon oxynitride; wherein the second defect/trapdensity is higher than the first defect/trap density.
 11. The method asset forth in claim 10 wherein the second layer of silicon oxynitride hasa defect/trap density gradient that extends from a third defect/trapdensity at a bottom of the second layer of silicon oxynitride to afourth defect/trap density at a top of the second layer of siliconoxynitride; wherein the fourth defect/trap density is higher than thethird defect/trap density.
 12. A method for reducing charge leakagethrough a layer formed on top of a polysilicon gate of a memory cell,the method comprising the steps of: forming a portion of the layer ofthe memory cell from a layer of silicon oxynitride, the layer of siliconoxynitride having a defect/trap density gradient that extends from abottom of the layer of silicon oxynitride to a top of the layer ofsilicon oxynitride; wherein the defect trap density gradient has a firstdefect/trap density at the bottom of the layer of silicon oxynitride anda second defect/trap density at the top of the layer of siliconoxynitride; and wherein the second defect/trap density is higher thanthe first defect/trap density.
 13. The method as set forth in claim 12wherein the defect/trap density gradient is formed by continuouslychanging a gas flow ratio of silane to ammonia during a time interval inwhich the layer of silicon oxynitride is deposited.
 14. The method asset forth in claim 12 wherein the layer of silicon oxynitride inhibitselectron transport through the layer of silicon oxynitride.
 15. Themethod as set forth in claim 12 wherein forming the portion of thebackend layer does not change CMOS device parameters.
 16. The method asset forth in claim 12 wherein the defect/trap density gradient is formedby using a silane to ammonia gas flow ranging from (i) approximately onepart silane to approximately sixteen parts ammonia to (ii) approximatelyone part silane to approximately three parts ammonia.
 17. A method forreducing charge leakage through a layer formed on top of a polysilicongate of a memory cell, the method comprising the steps of: depositing afirst layer of silicon oxynitride to form a first portion of the layerof the memory cell; and depositing a second layer of silicon oxynitrideto form a second portion of the layer of the memory cell; wherein thesecond layer of silicon oxynitride has a higher defect/trap density thandoes the first layer of silicon oxynitride.
 18. The method as set forthin claim 17 wherein the first layer of silicon oxynitride inhibitselectron transport through the first layer of silicon oxynitride. 19.The method as set forth in claim 17 wherein the second layer of siliconoxynitride provides protection from plasma induced damage.
 20. Themethod as set forth in claim 17 wherein the first layer of siliconoxynitride is deposited using a silane to ammonia gas flow comprisingapproximately one part silane to approximately sixteen parts ammonia.21. The method as set forth in claim 17 wherein the second layer ofsilicon oxynitride is deposited using a silane to ammonia gas flowcomprising approximately one part silane to approximately three partsammonia.